Photochemical and oxidative cyclisation of tetraphenylpyrroles

Abstract

The photochemical and oxidative cyclodehydrogenation reactions of tetraphenylpyrroles act in a complementary fashion for the cyclisation of N-ethyl and N-benzyl derivatives. In the case of the former, a doubly cyclised product was isolated from cyclisation with solid FeCl, while the latter gives a rearranged 3H-pyrrole upon irradiation.

Predicting the Outcome of Photocyclisation Reactions: A Joint Experimental and Computational Investigation

“…In previous work, we have experimentally characterised the preferred constitutional isomer produced upon photocyclisation of N ‐ethyl‐2,3,4,5‐tetraphenylpyrrole ( 6 b ) . Our current results show that the reaction pathway with the highest total aromatisation forces is the same pathway that leads to the experimentally observed product, as illustrated in Figure .…”

“…The protons of the N‐alkylating group were also affected, with the protons of the attaching methylene shifted to ≈4.9 ppm (for 10 a – 10 d ) and ≈6.0 ppm (for 10 e – 10 h ) from ≈4.1 ppm and ≈5.3 ppm, respectively. These changes are indicative of the increased aromatic area of 10 , and the associated increase in the ring anisotropy …”

“…However, no evidence of cyclised product was observed via either NMR or ES‐MS. However, we have previously shown that dibenzo[ e,g ]indoles can themselves be formed by photocyclisation from appropriately substituted pyrroles, 6 …”

“…The protons of the N-alkylating group were also affected, with the protons of the attaching methylene shiftedt o% 4.9 ppm (for 10 a-10 d)a nd % 6.0 ppm (for 10 e-10 h)f rom % 4.1 ppm and % 5.3 ppm, respectively.T hese changes are indicative of the increased aro-matic area of 10,a nd the associated increasei nt he ring anisotropy. [54] We were able to grow single crystals of 10 c, 10 f and 10 g, and these werec haracterisedu sing single crystal X-ray diffraction, along with the structures of 7a, 7d,a nd 7h (see the Supporting Information). The molecules all crystallised with one molecule in the asymmetric unit, as exemplifiedb yt he structure of 10 f shown in Figure 3( see ESI for structures of 10 c and 10 g).…”

“…However,n oe videnceo fc yclised product was observed via either NMR or ES-MS. However,w e have previously shown that dibenzo[e,g]indoles can themselves be formed by photocyclisation from appropriately substitutedp yrroles, 6. [54] In total, we report reactionc onditions and outcomes for 40 differentp otentially photocyclisable systems in Ta ble 1, repre-senting ad iverser ange of molecular scaffolds and substitution patterns.…”